Television system



Oct. 31, 1939. R. s. HOLMES 2,178,234

' TELEVISION SYSTEM Filed Feb. 28, 1954 Second Detector Impedanc (Tube13) Input Signal an grid of /7'ube Sif not loaded.

Input Signal on 7 05052654 13 showing synchronizing impulse leveling.

INVENTOR:

Ralph/ 5'. Holmes BY R'T'T'ORN Y Patented Oct. 31, 1939 PATENT OFFICEramvrsrou SYSTEM Ralph S. Holmes, l-laddonfield, N. J., asslgnor toRadio Corporation of America, a corporation of Delaware ApplicationFebruary 28, 1934, Serial No. 713,279

4 Claims.

, My invention relates to television systems and it has particularrelation to methods and means for obtaining accurate synchronism betweenthe scanning operation at a transmitting station and .3 the scanningoperation at a receiver.

In substantially all television systems it is necessary to periodicallymodulate the carrier wave to peak values, recurring at the linefrequency and at the picture-frame frequency, for synchroi nizingpurposes. If a Nipkow disc is used at the transmitting station forscanning the transmitted view, these impulses may be introduced throughthe use of a series of auxiliary openings circumferentially disposedaround the periphery of the disc through which light from a fixed sourceperiodically passes during rotation of the disc to impinge upon aphotoelectric cell. The output current from the cell, after suitableamplifica- 'tion,-is utilized to modulate the carrier to peak values, inthe direction corresponding to dark portions of the view, between linesand frames of the said view, givingimpulses which, at the receiver, maybe utilized for synchronizing. As a rule, at the receiver, the impulsesare separated -35 from each other by means of frequency and waveshapediscriminating circuits and, thereafter, are made use of to maintain aplurality of local oscillators, respectively, in step therewith. Theseoscillators, when the receiver includes a cathode ray tube, control thehorizontal and vertical de fiection of the electron beam as it sweepsover the fluorescent screen to reproduce the received picture impulsesas a visible image.

In order that the received picture, as it appears upon the screen, shallhave smooth boundaries the synchronizing impulses impressed upon thedeflection oscillators must not vary in amplitude. Such variations inamplitude may be caused by dirt in the synchronizing openings in thescanning disc, by non-uniformity in the size of the openings, byselective fading of the carrier wave or they may be occasioned byvarious conditionsin the receiver itself. Irrespective of the causativefactors, however, if the impulses are not of substantially invariableamplitude the edges of the reproduced view are irregular and, underextreme conditions the edges may assume a saw-tooth contour that is veryobjectionable.

It is, accordingly, the principal object of my invention to provide atelevision receiving circuit that shall compensate irregularities in theamplitude of the synchronizing impulses, whether such irregularities areintroduced at the transmitter or at the receiver, and one that 'shalldeliver synchronizing impulses at constant amplitude to the localsynchronizing oscillators or to other utilization devices.

The foregoing objects, and other objects ancillary thereto, 1accomplish, in a preferred embodiment of my invention, by providingdevices 5 for cl pping off the peaks, so to speak; of the synchronizingimpulses before delivering them to the aforesaid utilization devices.Specifically, I make use of the fact that the grid-impedance of agrid-leak biased thermionic tube is extremely 10 low for positiveimpulses applied to the grid thereof and I connect such a tube, inparallel with a normally biased synchronizing frequency amplifying tube,across the output circuit of an amplifier tube that handles both pictureand syn- 15 chronizing frequencies. In the event, therefore, that thesynchronizing impulses drive the grid of the grid-leak biased tubepositive, the said tube functions substantially as a short circuitacross the output resistor of the preceding tube 20 and prevents thepeaks of the impulses from materially altering the potential of the gridof the synchronizing frequency amplifying tube.

The novel features that I consider characteristic of my invention areset forth with particu- 25 larity in the appended claims. The inventionitself, however, both as to its organization and its method ofoperation, together with additional objects and advantages thereof, willbest be understood from the following description of a specific 30embodiment, when read in connection with the accompanying drawing, inwhich:

Figure 1 is a diagrammatic view of a portion of a television receiverincluding a preferred embodiment of my invention, and 5 Figure 2 is agraph to which reference will be made in explaining the operation of myimproved circuit.

Referring now to Figure 1 of the drawing, a television receiver to whichmy invention may '40 advantageously be added is illustrated as being ofthe superheterodyne type, including a second detector I. Obviously, aswill hereinafter appear, receivers of other types may be utilized. Thestages preceding the second detector have not been illustrated sincethey are immaterial to the operation of my invention.

The output from the second detector is impressed upon the inputterminals of a synchro- 5O nizing frequency amplification channelexemplified in the drawing by a plurality of resistancecondenser coupledthermionic devices 3, 5 and I which may be of the screen grid type, asshown. The synchronizing amplification channel may in- 55 clude means(notshown) for removing the picture signals from the synchronizingsignals.

The output circuit of the synchronizing frequency channel includes achoke-coil 8, across 5 which the horizontal impulses appear and a seriesconnected capacitor il across which the vertical impulses, at acomparatively low frequency as compared with the horizontal impulses,appear. Suitable circuit connections are included whereby 10 theimpulses may be impressed upon the local oscillators, (not shown) tokeep them in step.

Utilizing only the circuit thus far described, the synchronizingimpulses appearing across the choke coil and the capacitor would vary inam- 15 plitude with variation in the output from the second detector. Inaccordance with my invention, therefore, I provide further means forcompensating such variation which, for purposes of convenience will bedescribed with respect to the go horizontal impulses.

The specific compensating means I prefer to utilize, as shown in thedrawing, is an unbiased picture-frequency amplifying tube i3, the inputcircuit of which is connected in parallel with 25 the input circuit ofthe synchronizing frequency amplifying tube 5 across the output resistori5 of the first thermionic tube 3 in the synchronizing frequencychannel. The output circuit of the picture-frequency amplifying tube isresistor-ca- 3o pacitor coupled to the input circuit of the receivingcathode ray tube H which need not be illustrated in detail since itsstructure is now well known to those skilled in the art.

Referring now to Figure 2 of the drawing, the

5 picture-freqency amplifying tube i3 is not supplied with a fixed biasbut operates at a bias determined by the amplitude of the synchronzingimpulses in the positive direction with respect to their alternatingcurrent axis. Such operating bias is indicated in the drawing by theposition 40 of the line B and is determined by the grid-current drawn bythe tube when the synchronizing impulses are applied thereto. Morespecifically stated, the bias is a function of the capacity of thecoupling capacitor l9 and the resistance of u the grid-leak 2! connectedacross the input terminals of the picture frequency amplifying tube. Thecapacitor should be sufficiently large to hold substantially all of itscharge over the periods between successive horizontal synchronizingimpulses, thus assuming a working potential which will bias the tube toa good operating point for the picture signal and perhaps 75% of thesynchronizing impulses. The picture frequency amplifying tube l3, byreason of its low gridimpedance to positive impulses in excess of thepotential coresponding to the lower bend in the characteristicgrid-impedance curve, will then load the plate circuit of precedingamplifying tube 3 during the occurrence of the synchronizing im- 60pulse peaks and will level the impulses off to perhaps 75% of theaverage amplitude of all of the synchronizing impulses.

In a satisfactorily operating system constructed according to myinvention the coupling condenser 66 It has a value of .01 mid. and thegrid-leak of the picture frequency amplifying tube a magnitude of 2megohms.

In order that the vertical synchronizing impulses shall not disturb theoperation of the sys- 70 tern, it is desirable, in general, that theiraverage amplitude shall not exceed 75% of the average amplitude of thehorizontal synchronizing impulses. This may be accomplished throughproper adjustment of the synchronizing impulse 7i generators at thetransmitting station.

It might also be noted at this point that I have not found it necessaryto provide a separate tube to level of! the vertical synchronizingimpulses. Such a tube, however, could be provided subsequent to theoutput circuit of the synchronizing 5 frequency amplification channel.

From a consideration of the foregoing it will be apparent that I haveprovided a simplified system for clipping the peaks of the synchronizingimpulses in a television receiver. In addition, my 10 improved systemhas the further advantage that the picture-frequency amplifying tube isautomatically operated at the point on its characteristic curve where itgives the best picture signal amplification.

Although I have chosen a specific embodiment of my invention forpurposes of explanation other modifications within the scope thereofwill be apparent to those skilled in the art. My invention, therefore,is not to be limited except insofar as is necessitated by the prior artand by the spirit of the appended claims.

I claim as my invention:

1. In a television receiving system, an amplifying device having anoutput circuit, an amplify- 5 channel adapted to handle synchronizingimpulses, an input circuit for said channel, a picture-frequencyamplifying tube having a control grid and a uni-potential cathode andhaving an input circuit constituted only by a grid-leak, said controlgrid being connected directly to said unipotential cathode through saidgrid leak and means for connecting both of said input circuits inparallel across the first mentioned output circuit, whereby the picturefrequency amplifying tube functions as a load across the first mentionedoutput circuit and serves to maintain constant the amplitude of positivesynchronizing impulses applied to the input circuit of the synchronizingimpulse amplifying channel.

2. In a television receiving system, an electric discharge tube havingan output circuit, an amplifier for amplifying synchronizing impulses,said amplifier having an input circuit, an electric discharge tubehaving an input circuit constituted only by a grid leak, and means forconnecting both of said input circuits across said output circuit, theconnection between said last input circuit and said output circuitincluding a condenser which has suflicient capacity to integratesuccessive synchronizing impulses.

3. In a television system of the type in which picture signals andsynchronizing signals are transmitted as a composite signal and in whichthe synchronizing signals are of greater amplitude than the maximum peakamplitude of picture signals of like polarity, a receiver comprising acathode-ray tube having a control electrode, a picture signal amplifiertube having a control grid and a cathode, an output circuit and an inputcircuit for said tube, said output circuit being coupled to said controlelectrode, said input circuit including a grid leak resistor connectedbetween said control grid and said cathode, a grid condenser, and meansfor impressing said picture signals and synchronizing signals upon saidinput circuit through said grid condenser with the synchronizing signalsin the positive direction whereby they periodically drive said controlgrid positive to charge said condenser and apply a negative biasingpotential to said grid between synchronizing impulses, the time constant of the circuit including said grid condenser and said grid leakresistor being such that the charge on said grid condenser holds over asubstantial amount between positive synchronizing impulses.

4. In a television system of the type in which picture signals andsynchronizing signals are transmitted as a composite signaland in whichthe synchronizing signals are of greater amplitude than the maximum peakamplitude of picture signals of like polarity, a receiver comprising acathode-ray tube having a control electrode, a picture signal amplifiertube having a control grid and a cathode, an output circuit and an.

input circuit for said tube, said output circuit being coupled to saidcontrol electrode, said input circuit including a grid leak resistorconnected between said control grid and said cathode,

a grid condenser, and means for impressing said picture signals andsynchronizing signals upon said input circuit through said. gridcondenser with the synchronizing signals in the positive directionwhereby they periodically drive said control grid positive to apply anegative biasing potential thereto between synchronizing impulses, thetime constant of the circuit including said grid condenser and grid leakresistor and RALPH S. HOLMES.

